Lifetime Assessment of the NEXT Ion Thruster

Ion thrusters are low thrust, high specific impulse devices with required operational lifetimes on the order of 10,000 to 100,000 hr. The NEXT ion thruster is the latest generation of ion thrusters under development. The NEXT ion thruster currently has a qualification level propellant throughput requirement of 450 kg of xenon, which corresponds to roughly 22,000 hr of operation at the highest throttling point. Currently, a NEXT engineering model ion thruster with prototype model ion optics is undergoing a long duration test to determine wear characteristics and establish propellant throughput capability. The NEXT thruster includes many improvements over previous generations of ion thrusters, but two of its component improvements have a larger effect on thruster lifetime. These include the ion optics with tighter tolerances, a masked region and better gap control, and the discharge cathode keeper material change to graphite. Data from the NEXT 2000 hr wear test, the NEXT long duration test, and further analysis is used to determine the expected lifetime of the NEXT ion thruster. This paper will review the predictions for all of the anticipated failure mechanisms. The mechanisms will include wear of the ion optics and cathode s orifice plate and keeper from the plasma, depletion of low work function material in each cathode s insert, and spalling of material in the discharge chamber leading to arcing. Based on the analysis of the NEXT ion thruster, the first failure mode for operation above a specific impulse of 2000 sec is expected to be the structural failure of the ion optics at 750 kg of propellant throughput, 1.7 times the qualification requirement. An assessment based on mission analyses for operation below a specific impulse of 2000 sec indicates that the NEXT thruster is capable of double the propellant throughput required by these missions.

[1]  Michael J. Patterson,et al.  Space Station Cathode Design, Performance, and Operating Specifications , 1998 .

[2]  Michael J. Patterson,et al.  NEXT Ion Propulsion System Development Status and Performance , 2007 .

[3]  O. Duchemin.  An Investigation of Ion Engine Erosion by Low Energy Sputtering , 2001 .

[4]  Michael J. Patterson,et al.  Next: NASA's Evolutionary Xenon Thruster development status , 2003 .

[5]  Timothy R. Sarver-Verhey.  28,000 Hour Xenon Hollow Cathode LifeTest Results , 1997 .

[6]  B. Thornber,et al.  Temperature distributions in hollow cathode emitters , 2004 .

[7]  Dan M. Goebel,et al.  Numerical simulation of two-grid ion optics using a 3D code , 2004 .

[8]  J. L. Power,et al.  Solutions for discharge chamber sputtering and anode deposit spalling in small mercury ion thrusters , 1975 .

[9]  George C. Soulas.  Improving the Total Impulse Capability of the NSTAR Ion Thruster With Thick-Accelerator-Grid Ion Optics , 2001 .

[10]  Fred Wilson,et al.  Overview of the NEXT Ion Propulsion System Program at Aerojet , 2005 .

[11]  Paul J. Wilbur,et al.  NEXT Ion Optics Simulation via ffx , 2003 .

[12]  John R. Anderson,et al.  An Overview of the Results from the 30,000 Hr Life Test of Deep Space 1 Flight Spare Ion Engine , 2004 .

[13]  I. Katz,et al.  Hollow Cathode and Keeper-region Plasma Measurements Using Ultra-fast Miniature Scanning Probes , 2004 .

[14]  James S. Sovey,et al.  Retention of Sputtered Molybdenum on Ion Engine Discharge Chamber Surfaces , 2001 .

[15]  Michael J. Patterson,et al.  NEXT Ion Thruster Performance Dispersion Analyses , 2007 .

[16]  W. R. Kerslake,et al.  Long lifetime hollow cathodes for 30-cm mercury ion thrusters , 1976 .

[17]  Michael J. Patterson,et al.  NEXT Ion Engine 2000 hour Wear Test Plume and Erosion Results , 2004 .

[18]  Lynn A. Arrington,et al.  Overview of Diagnostics for the NEXT Long Duration Test , 2005 .

[19]  John R. Brophy,et al.  Probabilistic Analysis of Ion Engine Accelerator Grid Life , 1993 .

[20]  John E. Foster,et al.  Wear Testing and Analysis of Ion Engine Discharge Cathode Keeper , 2005 .

[21]  Michael J. Patterson,et al.  NEXT Ion Engine 2000 Hour Wear Test Results , 2004 .

[22]  R. Doernera.  Sputtering yield measurements during low energy xenon plasma bombardment , 2007 .

[23]  James E. Polk,et al.  Extending hollow cathode life for electric propulsion in long-term missions , 2004 .

[24]  A. M. Shroff,et al.  Experimental study of impregnated-cathode behavior, emission, and life , 1980 .

[25]  Scott D. Kovaleski,et al.  Life Model of Hollow Cathodes Using a Barium Calcium Aluminate Impregnated Tungsten Emitter , 2001 .

[26]  Michael J. Patterson,et al.  NASA's Evolutionary Xenon Thruster (NEXT) Phase 2 Development Status , 2005 .

[27]  James E. Polk,et al.  Ion Engine Service Life Validation by Analysis and Testing , 1996 .

[28]  John R. Anderson,et al.  Experimental and Theoretical Analysis for Designing a Grid Clearing System for the NEXT Ion Propulsion System , 2005 .

[29]  John R. Brophy,et al.  Status of the Extended Life Test of the Deep Space 1 Flight Spare Ion Engine After 30,000 Hours of Operation , 2003 .

[30]  M. J. Mandell,et al.  Ion engine neutralizer erosion in lab and space , 2005 .

[31]  John E. Foster,et al.  Testing and Analysis of NEXT Ion Engine Discharge Cathode Assembly Wear , 2003 .

[32]  Michael J. Patterson,et al.  Design and fabrication of a flight model 2.3 kW ion thruster for the Deep Space 1 Mission , 1998 .

[33]  D. H. Yan,et al.  Plasma-surface interaction studies in the HL-1 tokamak , 1989 .

[34]  Timothy R. Sarver-Verhey.  Scenario for Hollow Cathode End-Of-Life , 2000 .

[35]  Michael J. Patterson,et al.  PERFORMANCE EVALUATION OF 40 CM ION OPTICS FOR THE NEXT ION ENGINE , 2002 .

[36]  Andrew Hoskins,et al.  Development of a Prototype Model Ion Thruster for the NEXT System , 2004 .

[37]  John R. Anderson,et al.  Post-test analysis of the Deep Space 1 spare flight thruster ion optics , 2004 .

[38]  Iain D. Boyd,et al.  GRID EROSION MODELING OF THE NEXT ION THRUSTER OPTICS , 2003 .

[39]  Kevin E. Witzberger,et al.  Deep Space Mission Applications for NEXT: NASA's Evolutionary Xenon Thruster , 2004 .

[40]  Jonathan L. Van Noord.  NEXT Ion Thruster Thermal Model , 2007 .

[41]  Keith Goodfellow,et al.  An experimental and theoretical analysis of the grid clearing capability of the NSTAR ion propulsion system , 1999 .

[42]  Ira Katz,et al.  Plasma Surface Interaction Studies for Next-Generation Ion Thrusters , 2004 .

[43]  Michael J. Patterson,et al.  A Review of Testing of Hollow Cathodes for the International Space Station Plasma Contactor , 2001 .